This invention relates to pneumatic servo boosters and, particularly to pneumatic servo boosters of the kind including a shell housing consisting of front and rear shells, a flexible diaphragm, a valve body slidably extending through an opening formed in the rear shell and cooperating with the diaphragm to partition the interior of the shell housing into front and rear chambers, a valve mechanism incorporated in the valve body and at least one reinforcement rod rigidly connected to the front and rear sheels respectively and extending through the interior of the shell housing.
The valve mechanism conventionally comprises a poppet valve cooperating with a valve seat formed on the valve body and with another valve seat formed on a plunger which, in turn, is connected to an input rod and is slidably mounted in the valve body. When the poppet valve is spaced from the valve seat of the valve body and is seated on the valve seat of the plunger, the two chambers in the housing are communicated with each other and are maintained at a first referential pressure such as a vacuum pressure, and when the poppet valve engages with the valve seat of the valve body and separates from the valve seat of the plunger, the communication between the two chambers is intercepted and a second referential pressure such as the atmospheric pressure is introduced into one of the chambers while the other chamber is maintained at the first referential pressure.
In the non-actuated condition of the servo booster of the aforementioned kind, the poppet valve is spaced from the valve seat of the valve body by a predetermined small clearance, and the poppet valve seats on the valve seat of the plunger. The clearance is effective in increasing the space between the poppet valve and the valve seat of the valve body in the return stroke of the servo booster, thereby improving the responsiveness of the servo booster in the return stroke. However, in actuating the servo booster it is necessary to firstly take up the clearance, i.e., the clearance constitutes an ineffective stroke in the actuating stroke of the servo booster, thereby deteriorating the responsiveness and the pedal feeling of the servo booster in the actuating stroke.